Further to Ice Maiden's post, I have an ex from back about 10 years ago who, while we were together, came out as bi to me.
I said that was fine and not a problem.
About a week later he came out as gay.
It was very hard at first , for both of us, but we are still great mates and I've lived and worked with him and some of his boyfriends and all fine. He's married to a man now.
It was after seeing the issues he faced I ended up getting involved with LGB work, initially to help him and it spiralled into sitting on national committees.
It's not to say at all that that is what may happen, I know guys who are bi and have been in long standing relationships with either sex, it's like any relationship, you have no idea how it might turn out.
There's no need to rush things, take your time and just see how things go and please try and tread carefully and try not to give him any neuroses about his sexuality stemming from your own feelings.